Transversely-excited film bulk acoustic resonator with periodic etched holes

ABSTRACT

There are disclosed acoustic resonators and method of fabricating acoustic resonators. An acoustic resonator includes a single-crystal piezoelectric plate having front and back surfaces, the back surface attached to a surface of a substrate except for portions of the piezoelectric plate forming a diaphragm spanning a cavity in the substrate. A conductor pattern on the front surface includes an interdigital transducer (IDT) with interleaved fingers of the IDT disposed on the diaphragm. A periodic array of holes is provided in the diaphragm.

RELATED APPLICATION INFORMATION

This patent claims priority from provisional patent application62/874,709, filed Jul. 16, 2019, entitled XBAR WITH SLANTED AND/ORPERFORATED MEMBRANE.

This patent is also a continuation in part of application Ser. No.16/689,707, entitled BANDPASS FILTER WITH FREQUENCY SEPARATION BETWEENSHUNT AND SERIES RESONATORS SET BY DIELECTRIC LAYER THICKNESS, filedNov. 20, 2019, which is a continuation of application Ser. No.16/230,443, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR,filed Dec. 21, 2018, now U.S. Pat. No. 10,491,192, issued Nov. 26, 2019,which claims priority from the following provisional patentapplications: application 62/685,825, filed Jun. 15, 2018, entitledSHEAR-MODE FBAR (XBAR); application 62/701,363, filed Jul. 20, 2018,entitled SHEAR-MODE FBAR (XBAR); application 62/741,702, filed Oct. 5,2018, entitled 5 GHZ LATERALLY-EXCITED BULK WAVE RESONATOR (XBAR);application 62/748,883, filed Oct. 22, 2018, entitled SHEAR-MODE FILMBULK ACOUSTIC RESONATOR, and application 62/753,815, filed Oct. 31,2018, entitled LITHIUM TANTALATE SHEAR-MODE FILM BULK ACOUSTICRESONATOR. All of these applications are incorporated herein byreference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to bandpass filters with high powercapability for use in communications equipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a pass-band or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is less than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size, and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies proposed for future communications networks.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. Radio accesstechnology for mobile telephone networks has been standardized by the3GPP (3^(rd) Generation Partnership Project). Radio access technologyfor 5^(th) generation mobile networks is defined in the 5G NR (newradio) standard. The 5G NR standard defines several new communicationsbands. Two of these new communications bands are n77, which uses thefrequency range from 3300 MHz to 4200 MHz, and n79, which uses thefrequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79use time-division duplexing (TDD), such that a communications deviceoperating in band n77 and/or band n79 uses the same frequencies for bothuplink and downlink transmissions. Bandpass filters for bands n77 andn79 must be capable of handling the transmit power of the communicationsdevice. The 5G NR standard also defines millimeter wave communicationbands with frequencies between 24.25 GHz and 40 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view and two schematic cross-sectionalviews of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1.

FIG. 3A is an alternative schematic cross-sectional view of the XBAR ofFIG. 1.

FIG. 3B is another alternative schematic cross-sectional view of theXBAR of FIG. 1.

FIG. 4 is a graphic illustrating a primary acoustic mode in an XBAR.

FIG. 5 is a plan view of an XBAR with periodic etched holes.

FIG. 6 is a graph of the conductance versus frequency for XBARs with andwithout periodic etched holes.

FIG. 7 is a plan view of another interdigital transducer with periodicetched holes.

FIG. 8 is a graph of the conductance versus frequency for XBARs with andwithout periodic etched holes.

FIG. 9 is a flow chart of a process for fabricating an XBAR withperiodic etched holes.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely-excited film bulk acousticresonator (XBAR) 100. XBAR resonators such as the resonator 100 may beused in a variety of RF filters including band-reject filters, band-passfilters, duplexers, and multiplexers. XBARs are particularly suited foruse in filters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having parallel front and backsurfaces 112, 114, respectively. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. In the examplespresented in this patent, the piezoelectric plates are Z-cut, which isto say the Z axis is normal to the front and back surfaces 112, 114.However, XBARs may be fabricated on piezoelectric plates with othercrystallographic orientations.

The back surface 114 of the piezoelectric plate 110 is attached to asurface of the substrate 120 except for a portion of the piezoelectricplate 110 that forms a diaphragm 115 spanning a cavity 140 formed in thesubstrate. The portion of the piezoelectric plate that spans the cavityis referred to herein as the “diaphragm” 115 due to its physicalresemblance to the diaphragm of a microphone. As shown in FIG. 1, thediaphragm 115 is contiguous with the rest of the piezoelectric plate 110around all of a perimeter 145 of the cavity 140. In this context,“contiguous” means “continuously connected without any interveningitem”. In other configurations, the diaphragm 115 may be contiguous withthe piezoelectric plate are at least 50% of the perimeter 145 of thecavity 140.

The substrate 120 provides mechanical support to the piezoelectric plate110. The substrate 120 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 114of the piezoelectric plate 110 may be bonded to the substrate 120 usinga wafer bonding process. Alternatively, the piezoelectric plate 110 maybe grown on the substrate 120 or attached to the substrate in some othermanner. The piezoelectric plate 110 may be attached directly to thesubstrate or may be attached to the substrate 120 via one or moreintermediate material layers (not shown in FIG. 1).

“Cavity” has its conventional meaning of “an empty space within a solidbody.” The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120 under the diaphragm 115. The cavity 140 may be formed, for example,by selective etching of the substrate 120 before or after thepiezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers overlap for a distance AP, commonly referred to asthe “aperture” of the IDT. The center-to-center distance L between theoutermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. As will be discussed in further detail, theprimary acoustic mode is a bulk shear mode where acoustic energypropagates along a direction substantially orthogonal to the surface ofthe piezoelectric plate 110, which is also normal, or transverse, to thedirection of the electric field created by the IDT fingers. Thus, theXBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that atleast the fingers of the IDT 130 are disposed on the diaphragm 115 ofthe piezoelectric plate which spans, or is suspended over, the cavity140. As shown in FIG. 1, the cavity 140 has a rectangular shape with anextent greater than the aperture AP and length L of the IDT 130. Acavity of an XBAR may have a different shape, such as a regular orirregular polygon. The cavity of an XBAR may have more or fewer thanfour sides, which may be straight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. A typical XBAR has more thanten parallel fingers in the IDT 110. An XBAR may have hundreds, possiblythousands, of parallel fingers in the IDT 110. Similarly, the thicknessof the fingers in the cross-sectional views is greatly exaggerated.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100.The piezoelectric plate 110 is a single-crystal layer of piezoelectricalmaterial having a thickness ts. ts may be, for example, 100 nm to 1500nm. When used in filters for LTE′ bands from 3.4 GHZ to 6 GHz (e.g.bands 42, 43, 46), the thickness ts may be, for example, 200 nm to 1000nm.

A front-side dielectric layer 214 may optionally be formed on the frontside of the piezoelectric plate 110. The “front side” of the XBAR is, bydefinition, the surface facing away from the substrate. The front-sidedielectric layer 214 has a thickness tfd. The front-side dielectriclayer 214 may be formed only between the IDT fingers (e.g. IDT finger238 b) or may be deposited as a blanket layer such that the dielectriclayer is formed both between and over the IDT fingers (e.g. IDT finger238 a). The front-side dielectric layer 214 may be a non-piezoelectricdielectric material, such as silicon dioxide or silicon nitride. tfd maybe, for example, 0 to 500 nm. tfd is typically less than the thicknessts of the piezoelectric plate. The front-side dielectric layer 214 maybe formed of multiple layers of two or more materials.

The IDT fingers 238 a and 238 b may be aluminum, an aluminum alloy,copper, a copper alloy, beryllium, gold, tungsten, molybdenum or someother conductive material. The IDT fingers are considered to be“substantially aluminum” if they are formed from aluminum or an alloycomprising at least 50% aluminum. The IDT fingers are considered to be“substantially copper” if they are formed from copper or an alloycomprising at least 50% copper. Thin (relative to the total thickness ofthe conductors) layers of other metals, such as chromium or titanium,may be formed under and/or over and/or as layers within the fingers toimprove adhesion between the fingers and the piezoelectric plate 110and/or to passivate or encapsulate the fingers and/or to improve powerhandling. The busbars (132, 134 in FIG. 1) of the IDT may be made of thesame or different materials as the fingers.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The geometry of the IDT of an XBAR differs substantially fromthe IDTs used in surface acoustic wave (SAW) resonators. In a SAWresonator, the pitch of the IDT is one-half of the acoustic wavelengthat the resonance frequency. Additionally, the mark-to-pitch ratio of aSAW resonator IDT is typically close to 0.5 (i.e. the mark or fingerwidth is about one-fourth of the acoustic wavelength at resonance). Inan XBAR, the pitch p of the IDT is typically 2 to 20 times the width wof the fingers. In addition, the pitch p of the IDT is typically 2 to 20times the thickness is of the piezoelectric plate 110. The width of theIDT fingers in an XBAR is not constrained to be near one-fourth of theacoustic wavelength at resonance. For example, the width of XBAR IDTfingers may be 500 nm or greater, such that the IDT can be readilyfabricated using optical lithography. The thickness tm of the IDTfingers may be from 100 nm to about equal to the width w. The thicknessof the busbars (132, 134 in FIG. 1) of the IDT may be the same as, orgreater than, the thickness tm of the IDT fingers.

FIG. 3A and FIG. 3B show two alternative cross-sectional views along thesection plane A-A defined in FIG. 1. In FIG. 3A, a resonator 300includes a piezoelectric plate 310 attached to a substrate 320. Aportion of the piezoelectric plate 310 forms a diaphragm 315 spanning acavity 340 in the substrate. The cavity 340 does not fully penetrate thesubstrate 320. Fingers of an IDT are disposed on the diaphragm 315. Thecavity 340 may be formed, for example, by etching the substrate 320before attaching the piezoelectric plate 310. Alternatively, the cavity340 may be formed by etching the substrate 320 with a selective etchantthat reaches the substrate through one or more openings (not shown)provided in the piezoelectric plate 310. In this case, the diaphragm 315may contiguous with the rest of the piezoelectric plate 310 around alarge portion of a perimeter of the cavity 340. For example, thediaphragm 315 may be contiguous with the rest of the piezoelectric plate310 around at least 50% of the perimeter of the cavity 340. Anintermediate layer (not shown), such as a dielectric bonding layer, maybe present between the piezoelectric plate 310 and the substrate 320.

In FIG. 3B, a resonator 300′ includes a piezoelectric plate 310 attachedto a substrate 320. The substrate 320 includes a base 322 and anintermediate layer 324 disposed between the piezoelectric plate 310 andthe base 322. For example, the base 322 may be silicon and theintermediate layer 324 may be silicon dioxide or silicon nitride or someother material. A portion of the piezoelectric plate 310 forms adiaphragm 315 spanning a cavity 340 in the intermediate layer 324.Fingers of an IDT are disposed on the diaphragm 315. The cavity 340 maybe formed, for example, by etching the intermediate layer 324 beforeattaching the piezoelectric plate 310. Alternatively, the cavity 340 maybe formed by etching the intermediate layer 324 with a selective etchantthat reaches the substrate through one or more openings provided in thepiezoelectric plate 310. In this case, the diaphragm 315 may becontiguous with the rest of the piezoelectric plate 310 around a largeportion of a perimeter of the cavity 340. For example, the diaphragm 315may be contiguous with the rest of the piezoelectric plate 310 around atleast 50% of the perimeter of the cavity 340 as shown in FIG. 3B.Although not shown in FIG. 3B, a cavity formed in the intermediate layer324 may extend into the base 322.

FIG. 4 is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 4 shows a small portion of an XBAR 400including a piezoelectric plate 410 and three interleaved IDT fingers430 which alternate in electrical polarity from finger to finger. An RFvoltage is applied to the interleaved fingers 430. This voltage createsa time-varying electric field between the fingers. The direction of theelectric field is predominantly lateral, or parallel to the surface ofthe piezoelectric plate 410, as indicated by the arrows labeled“electric field”. Due to the high dielectric constant of thepiezoelectric plate, the RF electric energy is highly concentratedinside the plate relative to the air. The lateral electric fieldintroduces shear deformation which couples strongly to a shear primaryacoustic mode (at a resonance frequency defined by the acoustic cavityformed by the volume between the two surfaces of the piezoelectricplate) in the piezoelectric plate 410. In this context, “sheardeformation” is defined as deformation in which parallel planes in amaterial remain predominantly parallel and maintain constant separationwhile translating (within their respective planes) relative to eachother. A “shear acoustic mode” is defined as an acoustic vibration modein a medium that results in shear deformation of the medium. The sheardeformations in the XBAR 400 are represented by the curves 460, with theadjacent small arrows providing a schematic indication of the directionand relative magnitude of atomic motion at the resonance frequency. Thedegree of atomic motion, as well as the thickness of the piezoelectricplate 410, have been greatly exaggerated for ease of visualization.While the atomic motions are predominantly lateral (i.e. horizontal asshown in FIG. 4), the direction of acoustic energy flow of the excitedprimary acoustic mode is substantially orthogonal to the surface of thepiezoelectric plate, as indicated by the arrow 465.

Considering FIG. 4, there is essentially no RF electric fieldimmediately under the IDT fingers 430, and thus acoustic modes are onlyminimally excited in the regions 470 under the fingers. There may beevanescent acoustic motions in these regions. Since acoustic vibrationsare not excited under the IDT fingers 430, the acoustic energy coupledto the IDT fingers 430 is low (for example compared to the fingers of anIDT in a SAW resonator) for the primary acoustic mode, which minimizesviscous losses in the IDT fingers.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. In such devices, the acoustic mode iscompressive with atomic motions and the direction of acoustic energyflow in the thickness direction. In addition, the piezoelectric couplingfor shear wave XBAR resonances can be high (>20%) compared to otheracoustic resonators. High piezoelectric coupling enables the design andimplementation of microwave and millimeter-wave filters with appreciablebandwidth.

FIG. 5 is a plan view of an XBAR 500 with periodic etched holes. TheXBAR 500 includes a piezoelectric plate 510 having parallel front andback surfaces 512, 514, respectively. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent.

The back surface 514 of the piezoelectric plate is attached to surfaceof a substrate 520. A portion of the piezoelectric plate forms adiaphragm spanning a cavity 540 in the substrate 520. As shown in FIG.5, the cavity 540 extends completely through the substrate 520. Thecavity may only extend part way through the substrate, as shown in FIG.3A and FIG. 3B.

An IDT 530 is formed on the surface of the piezoelectric plate 510. TheIDT 530 includes a first busbar 532 and a second busbar 534. A first setof parallel fingers, such as finger 536 extends from the first busbar532. A second set of parallel fingers extends from the second busbar534. The first and second sets of fingers are parallel and interleaved.At least the interleaved fingers of the IDT are disposed on thediaphragm. A periodic array of holes 580 are formed in the diaphragm. Asshown in FIG. 5, the periodic array includes one hole at the end of eachIDT finger. Specifically, a hole is disposed between the end of each ofthe first set of fingers and the second busbar and a hole is disposedbetween the end of each of the second set of fingers and the firstbusbar. Other periodic arrangements of the holes, such as at the ends ofalternate IDT fingers may be used.

The periodic array of holes 580 has two effects on the performance ofthe XBAR 500. First, the holes scatter, and thus inhibit resonance of,spurious acoustic waves traveling parallel to the IDT fingers. Suchspurious acoustic waves can introduce ripple in the input/outputtransfer function of XBAR filters. Second, the array of holes 580appears to increase the Q-factor of XBAR devices, possibly by helping toconfine the primary shear acoustic mode to the aperture of the XBAR.

As shown in FIG. 5, the holes 580 are right circular cylinders with adiameter approximately equal to the width of the IDT fingers. The sizeand shape of the holes in FIG. 5 is exemplary. The holes may be largeror smaller than the width of the IDT fingers and may have across-sectional shape other than circular. For example, thecross-sectional shape of the holes may be oval, square, rectangular, orsome other shape. The holes need not necessarily pass through thepiezoelectric plate. The holes may be blind holes that only extend partway though the thickness of the piezoelectric plate. The size and depthof the holes must be sufficient to create a domain with significantlyreduced acoustic impedance. An additional benefit of holes at the endsof the IDT fingers is reduction of parasitic capacitance between the IDTfinger tips and the adjacent busbar.

FIG. 6 is a graph of the conductance versus frequency for XBARs with andwithout periodic etched holes. The conductance was determined by3-dimensional simulation using a finite element technique. The solidline 610 is the conductance (on a logarithmic scale) of an XBAR withholes at the end of each IDT finger, as shown in FIG. 5. The dashed line620 is the conductance of a similar XBAR without holes. The improvementin the Q-factor is evident in the higher, sharper conductance peak tothe resonance frequency of 4.64 GHz. Above the resonance frequency,local variations, or ripple, in conductance are reduced, but noteliminated, by the presence of the array of holes.

FIG. 7 is a plan view of another XBAR 700 with periodic array of etchedholes. The XBAR 700 includes a piezoelectric plate 710, a substrate 720(not visible beneath the piezoelectric plate), an IDT 730, and a cavity740. Each of these elements is comparable to the corresponding elementof the XBAR 500 of FIG. 5, except that the upper and lower (as seen inthe figure) edges of the cavity and the busbars of the IDT are notperpendicular to the IDT fingers. Specifically, the upper and loweredges of the cavity and the busbars are inclined by an angle θ withrespect to a line perpendicular to the IDT fingers. The angle θ may bebetween 0 and 25 degrees for example.

FIG. 8 is a graph of the conductance versus frequency for two XBARs withperiodic etched holes. The conductance was determined by 3-dimensionalsimulation using a finite element technique. The solid line 810 is theconductance (on a logarithmic scale) of an XBAR with holes at the end ofeach IDT finger, as shown in FIG. 5. The dashed line 820 is theconductance of an XBAR with the busbars and upper and lower edges of thecavity not perpendicular to the IDT fingers and holes at the end of eachIDT finger, as shown in FIG. 7. The Q-factors of the two XBARs at theresonance frequency are comparable Above the resonance frequency, localvariations, or ripple, in conductance are further reduced in the devicewith the busbars and upper and lower edges of the cavity notperpendicular to the IDT fingers.

Description of Methods

FIG. 9 is a simplified flow chart showing a process 900 for making anXBAR or a filter incorporating XBARs. The process 900 starts at 905 witha substrate and a plate of piezoelectric material and ends at 995 with acompleted XBAR or filter. The flow chart of FIG. 9 includes only majorprocess steps. Various conventional process steps (e.g. surfacepreparation, cleaning, inspection, baking, annealing, monitoring,testing, etc.) may be performed before, between, after, and during thesteps shown in FIG. 9.

The flow chart of FIG. 9 captures three variations of the process 900for making an XBAR which differ in when and how cavities are formed inthe substrate. The cavities may be formed at steps 910A, 910B, or 910C.Only one of these steps is performed in each of the three variations ofthe process 900.

The piezoelectric plate may be, for example, Z-cut lithium niobate orlithium tantalate as used in the previously presented examples. Thepiezoelectric plate may be some other material and/or some other cut.The substrate may preferably be silicon. The substrate may be some othermaterial that allows formation of deep cavities by etching or otherprocessing.

In one variation of the process 900, one or more cavities are formed inthe substrate at 910A, before the piezoelectric plate is bonded to thesubstrate at 920. A separate cavity may be formed for each resonator ina filter device. The one or more cavities may be formed usingconventional photolithographic and etching techniques. Typically, thecavities formed at 910A will not penetrate through the substrate, andthe resulting resonator devices will have a cross-section as shown inFIG. 3A or FIG. 3B.

At 920, the piezoelectric plate is bonded to the substrate. Thepiezoelectric plate and the substrate may be bonded by a wafer bondingprocess. Typically, the mating surfaces of the substrate and thepiezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the substrate. One or both mating surfaces may be activatedusing, for example, a plasma process. The mating surfaces may then bepressed together with considerable force to establish molecular bondsbetween the piezoelectric plate and the substrate or intermediatematerial layers.

A conductor pattern, including IDTs of each XBAR, is formed at 930 bydepositing and patterning one or more conductor layer on the front sideof the piezoelectric plate. The conductor layer may be, for example,aluminum, an aluminum alloy, copper, a copper alloy, or some otherconductive metal. Optionally, one or more layers of other materials maybe disposed below (i.e. between the conductor layer and thepiezoelectric plate) and/or on top of the conductor layer. For example,a thin film of titanium, chrome, or other metal may be used to improvethe adhesion between the conductor layer and the piezoelectric plate. Aconduction enhancement layer of gold, aluminum, copper or other higherconductivity metal may be formed over portions of the conductor pattern(for example the IDT bus bars and interconnections between the IDTs).

The conductor pattern may be formed at 930 by depositing the conductorlayer and, optionally, one or more other metal layers in sequence overthe surface of the piezoelectric plate. The excess metal may then beremoved by etching through patterned photoresist. The conductor layercan be etched, for example, by plasma etching, reactive ion etching, wetchemical etching, and other etching techniques.

Alternatively, the conductor pattern may be formed at 930 using alift-off process. Photoresist may be deposited over the piezoelectricplate. and patterned to define the conductor pattern. The conductorlayer and, optionally, one or more other layers may be deposited insequence over the surface of the piezoelectric plate. The photoresistmay then be removed, which removes the excess material, leaving theconductor pattern.

At 940, a front-side dielectric layer may be formed by depositing one ormore layers of dielectric material on the front side of thepiezoelectric plate. The one or more dielectric layers may be depositedusing a conventional deposition technique such as sputtering,evaporation, or chemical vapor deposition. The one or more dielectriclayers may be deposited over the entire surface of the piezoelectricplate, including on top of the conductor pattern. Alternatively, one ormore lithography processes (using photomasks) may be used to limit thedeposition of the dielectric layers to selected areas of thepiezoelectric plate, such as only between the interleaved fingers of theIDTs. Masks may also be used to allow deposition of differentthicknesses of dielectric materials on different portions of thepiezoelectric plate.

In a second variation of the process 900, one or more cavities areformed in the back side of the substrate at 910B. A separate cavity maybe formed for each resonator in a filter device. The one or morecavities may be formed using an anisotropic or orientation-dependent dryor wet etch to open holes through the back-side of the substrate to thepiezoelectric plate. In this case, the resulting resonator devices willhave a cross-section as shown in FIG. 1.

At 950, periodic holes, as shown in FIG. 5 and FIG. 7, may be formed.The periodic holes may extend part way or completely through thepiezoelectric plate and the front-side dielectric layer, if present. Forexample, the positions of the holes may be defined photolithographicallyand the holes may be formed using a suitable wet or dry etching process.

In a third variation of the process 900, one or more cavities in theform of recesses in the substrate may be formed at 910C by etching thesubstrate using an etchant introduced through openings in thepiezoelectric plate. The periodic holes formed at 950 may serve as theopenings through which the etchant is introduced. A separate cavity maybe formed for each resonator in a filter device. The one or morecavities formed at 910C will not penetrate through the substrate, andthe resulting resonator devices will have a cross-section as shown inFIG. 3A or FIG. 3B.

In all variations of the process 900, the filter device is completed at960. Actions that may occur at 960 include depositing anencapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or aportion of the device; forming bonding pads or solder bumps or othermeans for making connection between the device and external circuitry;excising individual devices from a wafer containing multiple devices;other packaging steps; and testing. Another action that may occur at 960is to tune the resonant frequencies of the resonators within the deviceby adding or removing metal or dielectric material from the front sideof the device. After the filter device is completed, the process ends at995.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. An acoustic resonator, comprising: a single-crystalpiezoelectric plate having front and back surfaces, the back surfaceattached to a surface of a substrate except for portions of thepiezoelectric plate forming a diaphragm spanning a cavity in thesubstrate; a conductor pattern formed on the front surface, theconductor pattern comprising an interdigital transducer (IDT) withinterleaved fingers of the IDT disposed on the diaphragm; and a periodicarray of holes in the diaphragm.
 2. The acoustic resonator of claim 1,wherein the piezoelectric plate and the IDT are configured such that aradio frequency signal applied to the IDT excites a primary shearacoustic mode in the diaphragm.
 3. The acoustic resonator of claim 1,wherein the piezoelectric plate is one of lithium niobate and lithiumtantalate.
 4. The acoustic resonator of claim 1, wherein the periodicarray of holes consists of one hole proximate the end of every finger ofthe IDT.
 5. The acoustic resonator of claim 1, wherein the interleavedfingers comprise: a first set of parallel fingers extending from a firstbusbar; and a second set of parallel fingers extending from a secondbusbar, wherein the first and second sets of parallel fingers areinterleaved and disposed on the diaphragm.
 6. The acoustic resonator ofclaim 5, wherein the periodic array of holes comprises: one hole betweenthe end of each of the first set of parallel fingers and the secondbusbar; and one hole between the end of each of the second set ofparallel fingers and the first busbar.
 7. The acoustic resonator ofclaim 5, wherein the first and second busbars are perpendicular to theinterleaved fingers.
 8. The acoustic resonator of claim 5, wherein thefirst and second busbars are inclined by and angle θ with respect to adirection perpendicular to the interleaved fingers, the angle θ beinggreater than 0 and less than or equal to 25 degrees.
 9. The acousticresonator of claim 1, wherein each of the periodic array of holes extendthrough the diaphragm.
 10. The acoustic resonator of claim 1, whereineach of the periodic array of holes is a blind hole not extendingthrough the diaphragm.
 11. A method for fabricating an acousticresonator on a single-crystal piezoelectric plate having a front surfaceand a back surface attached to a substrate, comprising: creating acavity in the substrate such that a portion of the piezoelectric plateforms a diaphragm spanning the cavity; forming a conductor pattern onthe front surface, the conductor pattern comprising an interdigitaltransducer (IDT) with interleaved fingers of the IDT disposed on thediaphragm; and forming a periodic array of holes in the diaphragm. 12.The method of claim 11, wherein the piezoelectric plate and the IDT areconfigured such that a radio frequency signal applied to the IDT excitesa primary shear acoustic mode in the diaphragm.
 13. The method of claim11, wherein the piezoelectric plate is one of lithium niobate andlithium tantalate.
 14. The method of claim 11, wherein forming theperiodic array of holes comprises: forming one hole proximate the end ofevery finger of the IDT.
 15. The method of claim 11, wherein theinterleaved fingers comprise: a first set of parallel fingers extendingfrom a first busbar; and a second set of parallel fingers extending froma second busbar, wherein the first and second sets of parallel fingersare interleaved and disposed on the diaphragm.
 16. The method of claim15, wherein forming the periodic array of holes comprises: forming onehole between the end of each of the first set of parallel fingers andthe second busbar; and forming one hole between the end of each of thesecond set of parallel fingers and the first busbar.
 17. The method ofclaim 15, wherein the first and second busbars are perpendicular to theinterleaved fingers.
 18. The method of claim 15, wherein the first andsecond busbars are inclined by and angle θ with respect to a directionperpendicular to the interleaved fingers, the angle θ being greater than0 and less than or equal to 25 degrees.
 19. The method of claim 11,wherein forming the periodic array of holes comprises: forming holesextending through the diaphragm.
 20. The method of claim 11, whereinforming the periodic array of holes comprises: forming blind hole notextending through the diaphragm.